In an exhaust turbocharger having a relatively small size used for an internal combustion engine of an automobile or the like, employed is a structure where the exhaust gas from the engine is used to fill a scroll formed inside a turbine housing. The exhaust gas is then let through a plurality of nozzle vanes disposed on the inner circumferential side of the scroll so as to act on the turbine rotor disposed on the inner circumferential side of the nozzle vanes. A variable geometry turbocharger which includes a variable nozzle mechanism having a plurality of nozzle vanes capable of varying their blade angles has been widely used.
FIG. 8 is an illustration of an example of a variable geometry turbocharger having a variable nozzle mechanism. FIG. 8 is a partial cross-sectional view taken along the rotational shaft center K. As illustrated in the drawing, the variable geometry turbocharger 1 includes a turbine housing 3. A scroll 5 is formed into a spiral shape on the outer circumferential part on the upstream side of the turbine housing 3. On the other hand, a turbine wheel 7 is disposed on the inner circumferential part of the turbine housing 3. The rotational center of the turbine shaft 9, to which the turbine wheel 7 is mounted, is the rotational shaft center K and coaxial with a compressor (not illustrated). The turbine shaft 9 is rotatably supported by a bearing housing 13 via a bearing 11.
A recess of an annular shape is formed on the back face of the bearing housing 13. This recess houses a variable nozzle mechanism 19 being a nozzle assembly including nozzles 15, a nozzle mount 17, a nozzle plate 18, etc. Each nozzle 15 has a nozzle vane 15a and a nozzle shaft 15b. A plurality of nozzle vanes 15a are disposed around the rotational shaft center K at predetermined intervals at the inner circumferential side of the scroll 5 so as to surround the outer circumference of the turbine wheel 7 at a position relative to the radial direction of the turbine. The nozzle shafts 15b are supported on the nozzle mount 17 fixed to a bearing housing 13 so as to be rotatable around the movable center C. The variable nozzle mechanism 19 is configured such that the blade angles of the nozzle vanes 15a are varied upon receiving rotational force from an actuator via a link mechanism 21.
FIG. 9 is a cross-sectional view taken along the line A-A as seen in the direction of the arrows of FIG. 8. As illustrated in FIG. 9, a nozzle throat 23 is formed between one of the nozzle vanes 15a and adjacent one of the nozzle vanes 15a. The exhaust gas G flows into the turbine wheel 7 through the nozzle throat 23. The opening degree of the flow path of the nozzle throat 23 is controlled by rotation of the nozzle shaft 15b of the nozzle vane 15a. The nozzle shaft 15b is controlled to rotate in the closing direction that closes the nozzle throat 23 when the flow rate of the exhaust gas is low, while it is controlled to rotate in the opening direction that opens the nozzle throat 23 when the flow rate of the exhaust gas is high.
Further, as illustrated in FIG. 9, across the nozzle vanes 15a and the nozzle throats 23 formed between the nozzle vanes 15a, the scroll 5 side being the outer circumferential side becomes the high pressure side H and the turbine wheel 7 side being the inner circumferential side becomes the low pressure side U due to the exhaust gas G injected as a drive fluid.
Accordingly, due to the pressure difference between the static pressure acting on the pressure surface 25 of the nozzle vane 15a adjacent to the high pressure side H and the static pressure acting on the suction surface 27 adjacent to the low pressure side U, a moment M (−) is generated in the closing direction of the nozzle throat 23 at the leading edge 29 side of the nozzle vane 15a, while a moment M (+) is generated in the opening direction at the trailing edge 31 side, the moments M (−) and M (+) acting on the nozzle vane 15a around the nozzle shaft 15b as the center of rotation.
If the balance of the moment M (−) in the closing direction acting on the leading edge 29 side caused by the static pressure and the moment M (+) in the opening direction acting on the trailing edge 31 side fluctuates to switch the directions of the moments acting on nozzle vane 15a, hysteresis occurs due to the backlash of the link mechanism 21 or the like that operates the variable nozzle mechanism 19, which raises an accuracy problem in the nozzle control. Further, there has been a problem that, in the case in which the moment M (−) in the closing direction is larger, a trouble in the drive system such as the actuator of the variable nozzle mechanism 19 leads to closure of the nozzle path, stopping operation of the variable nozzle mechanism 19.
In view of the above problems, JP2009-517578 (Patent Document 1) and JP2003-254074 (Patent Document 2) disclose modifying the shapes of the upper and lower surfaces of the vanes to adjust the rotation moments acting on the nozzle vanes caused by the pressure of the operating fluid acting on the upper and lower surfaces of the vanes.